Chlorine in residual amounts is usually present in the outlet or offgas streams following normal chlorine recovery operations. The need for a chlorine recovery system can occur in a number of ways, for instance, in caustic-chlorine plants wherein aqueous salt solutions are electrolyzed to produce chlorine and caustic soda, or plants wherein fused salt is electrolyzed to chlorine and sodium metal. In general, such plants include those in which a process is operated wherein a metallic halide undergoes electrolysis to produce free chlorine and/or other products. The necessity for special chlorine recovery arises from the fact that while approximately 95-96% of the chlorine can be liquefied and recovered in condensers, the remaining 4 to 5% passes out with the noncondensable gases. In most chlorine producing plants, this so-called "sniff gas" is neutralized with caustic, or further processed to recover chlorine.
Pressure swing adsorption (PSA) processes provide an efficient and economical means for separating a multicomponent gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbed gas can be an impurity which is removed from the less strongly adsorbed gas which is taken off as product, or, the more strongly adsorbed gas can be the desired product which is separated from the less strongly adsorbed gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feedstream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbed gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In PSA processes, a multicomponent gas is typically passed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, i.e. the more strongly adsorbed components, while at least one other component passes through, i.e. the less strongly adsorbed components. At a defined time, the passing of feedstream to the adsorber is terminated and the adsorption zone is depressurized by one of more cocurrent depressurization steps wherein the pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure in the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. Such PSA processing is disclosed in U.S. Pat. No. 3,430,418, issued to Wagner, U.S. Pat. No. 3,564,816, issued to Batta, and in U.S. Pat. No. 3,986,849, issued to Fuderer et al., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system.
As noted above the more strongly adsorbed components, i.e., the adsorbates, are removed from the adsorber bed by countercurrently depressurizing the adsorber bed to a desorption pressure. In general, lower desorption pressures are preferred in order to provide more complete removal of the adsorbates during the desorption step.
The sniff gas, or chlorine plant offgas is composed principally of air, hydrogen and chlorine, the chlorine commonly being present in amounts up to approximately 60% by volume. The chlorine content of the sniff or residual gas can be separated by adsorption on various solid adsorbents, but some of these adsorbents present practical problems which limit their commercial value in combination with chlorine plants. Two solid adsorbents which have been considered are activated carbon and silica gel. Highly activated carbon is known to catalyze the reaction between hydrogen and chlorine to produce hydrochloric acid which is undesirable. Less active carbon has an extremely low adsorption capacity which results in large amounts of adsorbent.
A U.S. Pat. No. 1,617,305 to Guyer teaches that the thermal conductivity of silica gel is so low that for separating the gas mixtures containing high proportions of chlorine, the ratio of the adsorption period to the period required to desorb the chlorine from the silica gel is inordinately small. The process of Guyer subjects the gas mixture to treatment with silica gel in a series of adsorbers at superatmospheric pressure and desorbs the chlorine at a lower pressure while the temperature is maintained low enough to provide a useful margin in content of chlorine adsorbed on the silica gel. The pressure of the desorption step is maintained much lower than atmospheric pressure and the desorption temperature is maintained much lower than atmospheric temperature and preferably is refrigerated, i.e., not higher than about 10.degree. C. throughout the adsorption and desorption cycle.
U.S. Pat. No. 2,800,197 to Wynkoop teaches a process for the recovery of chlorine from sniff gas wherein residual chlorine is recovered in a continuous manner from a two zone adsorption process with interzone cooling. The adsorption zones contain silica gel. At least a portion of the recovered chlorine-free gas stream is employed to strip the final traces of adsorbed chlorine during the regeneration cycle.
One more recent example of a process which uses adsorption to recover chlorine is a Japanese publication, JP 04362002-A, which discloses a process for the recovery of liquid chlorine from a gas mixture. The process includes the steps of compressing and condensing of the mixture to provide a liquid chlorine stream and a non-condensing gas. The non-condensing gas is further processed in an adsorption tower to produce a vent gas stream containing up to 1 vol-% chlorine. Upon desorption at lower pressure, the desorbed gas comprising adsorbed chlorine is returned to the compression step.
In a typical chlor-alkali plant, a chlorine plant offgas stream is produced which contains up to 60 volume percent chlorine. Conventional technology which generally comprises multistage offgas compression, cooling, and partial condensation is followed by an absorption and stripping process which uses a carbon tetrachloride solution to produce a vent gas stream containing about 1 vol % chlorine. One example of a process which uses the toxic solvent carbon tetrachloride is disclosed in German Patent DD-226862-B, wherein a chlorine containing gas mixture is contacted with a solvent in an absorber, formed as a bubble column which has been cooled to a temperature to between -20.degree. to -25.degree. C. The solvent is a mixture of chloro-hydrocarbons containing 60-80% carbon tetrachloride. The solvent is subsequently desorbed to provide a desorbed gaseous chlorine. Some traces of the carbon tetrachloride are entrained in the desorbed gaseous chlorine. The vent gas from the absorption and stripping process is further followed by a caustic scrubbing step to neutralize and remove any remaining chlorine in the plant offgas stream. This caustic scrubbing step produces a liquid stream containing chlorine which must be disposed or sold as a low value laundry bleach.
Processes for the remediation of chlorine plant offgas are sought which eliminate the use of potentially toxic solvents.
Processes for the remediation of chlorine plant offgas are sought which complement and preferably replace existing solvent absorption and stripping processes.
Processes for the remediation of chlorine plant offgas are sought which can reduce the chlorine content in the remediated gas, or vent gas to a level below 10 ppm-vol. Furthermore, processes are sought which eliminate the need for the caustic scrubbing step by reducing the chlorine content in the vent gas to less than 1 ppm-wt chlorine.
Processes are sought which reduce the adsorbent inventory by permitting the adsorption and regeneration to take place rapidly.